One of methods used by a transmitter to reliably transmit information to a receiver is a method of performing retransmission at a physical layer. The method of performing retransmission at the physical layer is technology that, even when the receiver cannot obtain information that has been transmitted, reliably provides the information to the receiver by transmitting some data that is based on the information again. When the number of retransmissions increases, data transmission efficiency decreases. Therefore, it is important to realize a retransmission method that can transmit information to the receiver while keeping the number of retransmissions small. Note that the following describes general retransmission. When the receiver cannot properly decode some information (or packets) received from the transmitter, the transmitter transmits data corresponding to the information (or packets) again in response to a retransmission request made by the receiver. However, the transmitter may transmit, to the receiver, data that is different from the initially transmitted information (or packets), in response to the retransmission request as long as the transmitted information (or packets) can be restored with use of the data. In the present Description, the retransmission includes a case where the transmitter transmits, in response to the retransmission request, data that is different data from information (or packets) initially transmitted to the receiver and is data with which the initially transmitted information (or packets) can be restored.
FIG. 27 shows an exemplary retransmission method recited in Non Patent Literature 1. In FIG. 27, each Xi indicates information at a time point i, and each Pi indicates a parity at the time point i. When a coding rate is ½, a transmit sequence is composed of the information pieces Xi and the parities Pi. When the coding rate is ⅔, the transmit sequence is composed of bits excluding P2, X3, P6, X7, . . . , P2k+4, X3k+4, . . . (puncture), for example.
The transmitter initially transmits, to the receiver, the transmit sequence that is composed according to a method using the puncture at the coding rate of ⅔. Then, upon receiving a request from the receiver for retransmission, the transmitter transmits, to the receiver, bits that have not been initially transmitted (i.e. P2, X3, P6, X7, . . . , P2k+4, X3k+4, . . . ).
The receiver performs decoding at the coding rate of ½, with use of a received log likelihood ratio of the initially transmitted bits and a reception likelihood ratio of the retransmitted bits. In this case, the coding rate is lower at the time of the decoding for the retransmission than at the time of the initial decoding. Therefore, a packet error is less likely to occur at the time of retransmission.